Macro-based declarative config

This would sit fully on top of the existing imperative ConfigReader API.

Open to ideas, we might even want to coordinate with the new Codable effort: https://forums.swift.org/t/the-future-of-serialization-deserialization-apis/78585 - as the property macros might have similar requirements.

Start by reviewing the existing imperative ConfigReader API and the linked Swift Codable discussion. Clarify the macro-based declarative configuration design and how it should build on ConfigReader; done requires an agreed, implementable scope rather than an open-ended set of ideas.

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